Magneto and Toad are prisoners on the Stranger's asteroid, after he took them as specimens in X-Men #11 (and recaptured Magneto in X-Men #18). Magneto can detect a message beamed by magnetic rays.
The message comes from a castle on Earth where scientist Dane Whitman is trying to make up for his uncle Nathan Garrett's career as the villainous Black Knight (one of the Masters of Evil who bedevilled the Avengers between #6 and #16). Garrett died after falling from his flying horse while fighting Iron Man in Tales of Suspense #73, and Whitman inherited his castle in the US.
Meanwhile Captain America resigns from the Avengers because he is going to quit being Cap. This upsets Hawkeye and makes him snap at Natasha, who misinterprets it as being that he doesn't love her so much now she's given up being Black Widow (in #45). Goliath and Wasp go on vacation to Las Vegas in her new private jet (another product of the millions she inherited in #43), chauffeured to the airport by Charles, whom they don't realise is the villain Whirlwind who attacked them last issue. And Hercules goes to confront his father Zeus in Mount Olympus, but finds it deserted. This leaves only Quicksilver and Scarlet Witch back at Avengers Mansion.
Dane Whitman has received a reply from space to his magnetic broadcast. But his assistant Norris knocks him out, intending to take the credit for the first interstellar contact by magnetic rays. Unfortunately for him Magneto has used the magnetic beam to transport himself and Toad back to Earth. Norris tries to send them back, but Magneto renders him unconscious too.
Magneto intends to reform the Brotherhood of Evil Mutants. He'll leave Mastermind till later, but wants to recruit Wanda and Pietro immediately. He recalls how he rescued them from a mob or thought Wanda was a real witch. Now he uses Whitman's equipment to broadcast a coded message which the twins pick up on Avengers' monitors.
They follow directions to Garrett Castle, where Magneto tests them by animating a giant metal statue armed with a mace. When he asks them to return to his Brotherhood they refuse, telling him they are Avengers. He attacks them with a horde of armed normal-sized suits of armour. Quicksilver and Scarlet Witch fight back, but Magneto knocks them out.